The present invention, in some embodiments thereof, relates to Computed Tomography (CT) scanners applied, by way of example, to imaging of the cardiovascular system. More particularly, the invention relates to CT scanning of subjects undergoing intravenous injection of a contrast enhancement agent.
A CT scanner typically comprises an X-ray source and detector system, both mounted on a gantry arranged to rotate about a scanned subject. Attenuation data are collected for X-rays that have traversed the scanned subject and impinged on the detector system. The data are then computer-processed (or reconstructed) to generate images of axial cross sections in the scanned subject or to form volumetric 3D images, to display the images and to store the images on computer media.
In some CT systems, the subject support is arranged to move in the direction of the axis of rotation relative to the rotating gantry to provide a spiral mode of operation. The availability of spiral mode, fast gantry rotation and large area detectors has led to the development of Computed Tomography Angiography (CTA) scanning whereby CT is used to image the human cardiovascular system with the aid of intravenous (IV) contrast agent injection. CTA scanning is also possible on cone beam CT scanners wherein the beam is wide enough to cover the organ of interest e.g. brain or heart. In cone beam scanners the imaging of the whole organ may be accomplished at a single patient position relative to the gantry.
The contrast agent is typically injected by a programmable power injector where the duration of injection, the volumetric rate of injection, and other parameters are adjusted according to the characteristics of the patient and the scan protocol. Nevertheless, the rate at which contrast agent concentration builds up at a particular blood vessel is variable from person to person because of different body size, weight and hemodynamic parameters. Since it is desired to perform the actual CT scan as near to the time of maximum contrast agent concentration as possible, the delay between start of contrast agent injection and the CT scan will vary from subject to subject. Therefore, it is customary to determine the optimal start time of the diagnostic scan relative to the start of injection for each subject individually.
According to one method known in the art as “test bolus”, a preliminary injection of a small volume of contrast agent i.e., a test bolus, is used to determine the optimal timing. In this method the patient is injected with a small amount of contrast and a series of timed planar images are acquired in sequence at the same scan position. A region of interest (ROI) is graphically overlaid over a selected area of the image that exhibits contrast enhancement—for example, within the aorta—and the level of enhancement is determined as a function of time. The results are used to plan the delay between injection of the full required volume of contrast agent and the start of a subsequent diagnostic image data acquisition. Disadvantages of this method include the need for administration of additional contrast agent and the extra time and effort involved.
According to another method known in the art as “bolus tracking”, the gantry is set in motion, and a planar image is generated at a selected gantry rotational position and an ROI is overlaid on the planar image at a given axial patient position where contrast enhancement is visible—again, for example, within the aorta. Injection is pre-programmed and administrated. While the injection goes on, the scanner is used to generate a series of planar images at the given rotational position and the level of enhancement in the ROI is monitored in almost real time. The actual diagnostic scan starts at a time determined by the measured enhancement. The timing is derived for example from the rate of increase of the enhancement level or the time at which the contrast agent level reaches a pre-set threshold or by some other algorithm. However, it takes several seconds to terminate the “bolus tracking” scan and revert to normal CTA scan operation since the patient typically must be moved to a different position relative to the scanner, a beam collimator must be readjusted, and new scan parameters must be loaded into the acquisition, reconstruction and X ray systems. Thus, the actual diagnostic scan does not start exactly at a determined optimal time but rather at some estimated time. As CTA scan times become shorter with modern CT scanners, it becomes more important to improve the scan timing relative to injection. Such improvement is also important to reduce the amount of contrast agent that needs to be administrated.
General background on coronary CTA, and on conventional contrast agent monitoring may be found in Jacobs, How to Perform Coronary CTA. A to Z, Applied Radiology, December 2006 Supplement, the content of which is incorporated herein by reference as if fully disclosed.
Known CT scanners having multiple displaced X-ray sources have been found to have utility in connection with the present invention. Examples include U.S. Pat. No. 5,068,882 to Eberhard, and U.S. Pat. No. 6,760,399 to Malamud and published applications US 2006/285633 A1 to Sukovic et. al. and WO 2006/038145 A to Koken et. al., show devices of this kind, and are incorporated herein by reference as if fully disclosed.
Published application WO 2008/122971 to Dafni (corresponding to U.S. application Ser. No. 12/307,374 filed 5 Jan. 2009) owned by the assignee of the present application, shows multiple beam scanner arrangements. This application is also incorporated herein by reference as if fully disclosed.